<p>This study investigates the effect of the longitudinal reinforcement ratio, fiber content, and mixing water type on the shear strength of reinforced concrete (RC) beams containing seawater in their mix. Ten RC beams were tested with varying reinforcement ratios (1.0%, 1.4%, and 2.0%), chopped fiber contents (0, 0.5, 2.0, and 3.0 kg/m³), and mixing water types (freshwater and seawater). The results showed that increasing the reinforcement ratio from 1.0% to 2.0% enhanced the shear strength by 39%. The optimal fiber content of 2.0 kg/m³ increased the shear strength by up to 15% compared to the beam with no fibers. However, a further increase in fiber content to 3.0 kg/m³ did not significantly improve the shear strength, indicating a saturation point. When comparing beams mixed with seawater to those with freshwater, a 10% increase in shear strength was observed. The experimental shear strengths were evaluated against predictions from the CSA S806:12 (21), ACI 440.11–22, and JSCE-1997 design codes. The results revealed that all three codes provided conservative estimates. The CSA S806:12 (21) and JSCE-1997 codes had similar predictions, both underestimating the shear strength with a mean experimental-to-predicted ratio of 1.4. Meanwhile, ACI 440.11–22 showed the highest underestimation, with a ratio of 1.8. These findings highlight the need to refine existing design models to better capture the contribution of fiber content and seawater mixing to the shear behavior of GFRP-RC beams.</p>